The present invention relates to a method for controlling tones in a self-luminous display such as a plasma display panel and an electroluminescence display panel, and more particularly to a method for preventing false contouring.
There has been known an image display device having a self-luminous display panel such as a plasma display panel and an electroluminescence panel. The plasma display panel utilizes a gas discharge so that the quantity of the light emitted therefrom cannot be continually controlled. Hence the emission is actuated by pulses, that is, the brightness of the image on the display is represented by the number of the pulses, namely, by the frequency of the emission. The image becomes brighter as the number of the emission, or the frequency per unit time increases so that the tone can be controlled.
In order to drive the panel so as to show a picture thereon, each field of a composite video signal is divided into a plurality of sub-fields on a time axis. The sub-fields are differently weighted in order to impart a tone to the image on the display. Namely, a digital video signal is reproduced not by a dot sequential scanning of each pixel, but by repeating a plane sequential scanning of the pixel in accordance with the weight of the pixel.
As shown in FIG. 22a, each field is divided into eight sub-fields D.sub.8 to D.sub.1, corresponding to the 8 bits of a pixel data so that, in order to complete a field, a bit plane scanning takes place. The time length of each sub-field is determined in accordance with its weight. The ratio of weights from the first sub-field to the eighth sub-field are, for example, 128(2.sup.7):64(2.sup.6):32(2.sup.5):16(2.sup.4):8(2.sup.3):4(2.sup.2): 2(2.sup.1):1(2.sup.0), as shown in FIG. 22b. For example, when the logic value of the eighth position and hence the highest order bit of the pixel data is "1", which indicates the logic for emitting, the light is emitted 128 times during the sub-field D.sub.8. When the logic value of the eighth bit is "0", light is not emitted at all during the sub-field D.sub.8. When the seventh bit of the pixel data is "1", the light is emitted 64 times during the sub-field D.sub.7. When the plane sequential scanning is thus carried out eight times, the light from each pixel is visually recognized by the viewer as a brightness corresponding to the total of the pulses of eight sub-fields. Thus, the tone of 2.sup.8 (256) steps, from 0 to 255, can be obtained by combining the eight weights.
FIG. 22c shows, as examples, the light emitting periods corresponding to each sub-field of the eight-bit pixel data, "11111111", "10000000", and "100000001".
The above described sub-field system is an excellent system which enables to realize various tones in a single-tone display which is capable of indicating only two tones "1" and "0". However, a false contouring due to visual characteristics inherently occurs in the system. The false contouring is a phenomenon where a flat image, the levels of signals thereof cross the tone levels such as 128, 64, 32 and 16, which are the powers of 2. As a result, contour lines in stripes appear on the display as if the tones of the image are lost. The phenomenon becomes strikingly apparent when an image of a flat mass moves on the display and is hardly recognized when the image is stationary, that is when a still picture stored in a memory is shown. Namely, the false contours are recognized only when an image moves about level boundaries. In addition, when a load of a still visual signal fluctuates due to a noise included therein, the false contours also appear.
The cause of the false contouring is described with reference to FIGS. 23a, 23b, 24a, 24b, 24c and 24d. As shown in FIG. 23a, when the tone is decreased so that the number of pulses decreases from 128 to 127 in the next field, the emission at the sub-field D.sub.8 is stopped and the emission at the sub-fields D.sub.7 to D.sub.1 is started. The difference in the levels of the tone corresponds to 1 least significant bit (LSB). In such a case, a transition period t.sub.1 where the emission of light does not occur is so long that the viewer senses it as though the tone is decreased, although momentarily. As a result, there are formed on the display, stripes similar to isobaric curves in a weather map. Although the tone of each pixel is decreased only one step, since the stripes move with the movement of the image, they become apparent to the viewer.
FIG. 23b shows a case where bright contour lines are formed. When the number of pulses is increased from 127 to 128, emission at the sub-fields D.sub.7 to D.sub.1 is stopped and emission at the sub-field D.sub.8 is started. A transition period t.sub.2 is so short that the luminous density is increased. Hence a bright stripe appears on the display.
There are displays where the sub-field are arranged starting from the sub-field with a smallest weight as shown in FIGS. 24a to 24d. Each space before the sub-fields indicates a constant non-light emitting period for selecting the next sub-field at which the light is emitted. The spaces are omitted in FIGS. 23a and 23b for the sake of simplicity.
As shown in FIG. 24a when a pixel data is "11111111", corresponding to 255 pulses, the light is flashed during all of the sub-fields. When the pixel data is "10000000", corresponding to 128 pulses, the light is emitted only during the sub-field D.sub.8 (FIG. 24b). FIG. 24c shows an example where the data is "01111111" corresponding to 127 pulses, so that the light is emitted at sub-fields D.sub.1 to D.sub.7. When the data changes to "10000000", a transition period t.sub.5 shown in FIG. 24d becomes much longer than periods t.sub.3 and t.sub.4 which are shown respectively in FIGS. 24b and 24c. As a result, dark stripes appear on the display. To the contrary, when the data changes from "10000000" to "01111111", the non-light emitting period becomes short so that bright stripes appear.
In order to restrain the false contouring, Japanese Patent Application Laid-Open Nos. 2-291597, 3-145691 and 4-211294 propose to change the arrangement of the sub-fields. For example, the emitting period for a sub-field corresponding to the most significant bit (MSB) of the pixel data is positioned between those of the lower bits so that the difference in luminance, particularly that of the sub-field of the MSB is decreased. However, experiments have shown that the false contours are observed in the lower bit levels.